Photomasks used in the photolithography process for manufacturing integrated circuits (IC) are exposed to high levels of irradiance by deep ultraviolet (DUV) radiation, also known as actinic wavelength radiation. In recent years, a number of side-effects of the photolithography process at actinic wavelengths have been discovered, notably the degradation of photomask quality as a function of time and cumulative exposure. A common phenomenon is the growth of defects on mask surfaces, even when the masks were defect free when they were shipped from the mask supplier.
The mechanisms for progressive degradation of photomask quality are numerous. A partial list of the causes of degradation includes: growth of chemical contaminants on the photomask and on lens surfaces (often referred to as haze), chrome ion migration, modification of the photomask surface and bulk due to accumulated exposure to DUV radiation, and numerous other causes. Contaminants may form on the pellicle of the photomask, between the pellicle and the patterned mask, or on the mask itself. Contaminants on the mask may form on the photomask coating (e.g. chrome coatings, MoSi coatings), or on clear areas of the quartz substrate (pattern or scribe lines). The IC industry traditionally refers to the substrate material as glass or quartz, although it is typically made of fused silica.
Photomask transmission and phase changes that affect light passing through the photomask under some types of progressive degradation processes can be either positive or negative. Transmission can either increase or decrease, when no absorption is involved, as a result of optical thin-film effects or modifications of surface and bulk material properties. It is well known in the field of thin films optics that if a transparent thin film is deposited on a transparent substrate (such as, for example, glass), a change of reflection and transmission may result. For example, if the thickness of a film is one quarter of the wavelength of the incident radiation and the index of refraction of the film equals to the square-root of the index of the glass substrate, then reflection will drop to zero at normal incidence and transmission will increase by 4%-5%. When the value of the index of refraction value is not exactly the square root of the index of the substrate, or the thickness is less then a quarter of the wavelength, the transmission increase will be smaller. However, even at a fraction of a percent, the transmission increase will still be detectable.
In a lithography process that uses a photomask, local changes in DUV transmission and phase attributed to the photomask may result in changes of in the size of the printed features and in the critical dimension (CD) of the circuit elements, as well as in reduced printing contrast.
One well understood mechanism of progressive degradation, described by S. Shimada et al. in “A new model of haze generation and storage life-time estimation for masks” (BACUS news, April 2007, volume 23, issue 4), is the slow growth over time of ammonium sulfate, the growth being driven by the energy due to DUV exposure. Sulfate and ammonium ions (or other monomers) may be present on or near photomask surfaces as residual contamination from previous reticle cleaning processes, from airborne molecular contamination, or from materials released from the reticle or pellicle during mask manufacture or use. A chemical reaction between the reactant ions or seed molecules may be photochemically catalyzed by DUV exposure, nucleating small particles on photomask surfaces which later grow into larger particles. Such contaminants can also grow on the surfaces of the lenses of the projection lithography tools, locally reducing their transmission and, therefore, causing optical performance to deteriorate. Once contamination has grown to a critical threshold on the photomask, the pellicle must be removed, the mask cleaned, and a new pellicle reapplied prior to reuse of the photomask. When the mask cannot be cleaned, or fails to meet specifications after cleaning, the mask must be remade.
Another phenomenon, described by Tchikoulaeva et al. in “ACLV Degradation: Root Cause Analysis and Effective Monitoring Strategy” (Proc. of SPIE, PMJ April 2008, 7028-40) is that of the migration of chrome ions from the chrome edges of a mask to nearby clear quartz areas, creating an absorptive material comprising a mixture of Cr2O3 and Cr. This ion migration phenomenon is also enhanced by the accumulated exposure to DUV during the lithography printing process.
Haze growth and other factors causing transmission changes result in high cleaning costs, loss of productivity, and disruption of the delivery of photolithography products.
Defects in the photomask tend to grow slowly until a size threshold is reached. Once the threshold is reached, a rapid dramatic increase in the chemical haze often occurs. Failure to detect this increase immediately could result in poor performance or rejection of the integrated circuits printed on the silicon wafers. The threshold for fast growth is found to occur at a typical accumulated exposure of about 4000 Joules/cm2. The threshold, however, is dependent on a number of variable factors including the level of environmental contamination, details of the photomask manufacturing process, the storage environment of the photomask, etc. Thus, the inability to detect haze growth or other transmission changes at an early stage of their development could result in a serious reduction of yield when printing wafers.
Early detection of progressive changes in transmission has been found to be nearly impossible with existing photomask inspection tools. These tools are not sensitive to the changes in DUV transmission and reflection during the early stages of the process, typically less than 1%. Mapping of DUV transmission with high sensitivity is becoming essential even for unpatterned photomask blanks, coated or uncoated. As design rules of ICs become smaller and smaller, even transmission variations in the range of 0.1%-0.5% across a blank or a coated mask can measurably affect performance.
A system for measuring and mapping DUV transmission is also needed for use with the resolution enhancement techniques (RET) that are employed in photolithography. RETs, such as embedded phase shift masks (EPSM) and alternating phase shift masks (APSM), modify the phase of the DUV beam either by patterning an absorber such as MoSi (with EPSM), or by etching the quartz itself (with APSM). A MoSi or other phase shifting coating causes a change in the phase of a DUV beam in that depends on the thickness of the coating. A typical MoSi coating for a lithography tool with a 193-nm laser source is designed to yield a DUV transmission of 6%, equivalent to a phase shift of π (180°), relative to the fused silica substrate.
By measuring DUV transmission across a photomask and mapping it, a phase variation map can be generated by converting the transmission map to a map of optical thickness variation. Relative optical thickness is related to transmission by Beer's law for absorptive layers. Since phase change is a linear function of the optical thickness, the optical thickness, and therefore the transmission, can be converted to a phase shift. The thickness of a MoSi coating required to attain a phase shift of π at a printing wavelength of 193-nm is typically 70 nm. Such a coating transmits about 6% of the DUV incident beam. If other values of transmittance are required, other coating layers, such as SiO2 or Ta2O5, may be applied to the photomask in order to attain the required transmission and phase changes. DUV transmission mapping can therefore be translated into maps of phase and CD variations.
Transmission and phase changes due to contamination may occur suddenly, for example, after cleaning the mask. If the thickness of a MoSi or SiO2 layer changes by less then one percent during cleaning, phase and transmission changes could change the CD by more than 1 nanometer. Such small transmission variations in photomask blanks have also been observed, with larger variations observed in reclaimed masks that have had their pattern and coatings stripped away for reuse. Inspection tools based on optical image processing are not able to detect such small variations, where the change in transmission is less than 1%. Such tools typically employ imaging devices with small dynamic ranges, such as CCD cameras. Such imaging devices also typically have small fields of view that do not permit the statistical averaging needed to obtain a signal that exceeds the noise level.
Other fast optical methods, such as ellipsometry and scatterometry, can only be used to detect haze defects or tiny transmission variations where the defects take the form of thin films or other well-defined geometrical structures. However, as described by Jong Min Kim et al. in “Threshold residual ion concentration on photo-mask surface to prevent haze defects” (Photomask technology, Proceedings of SPIE, volume 6349, October 2006), such defects generally are not agglomerated into films or large particles during early stages of defect formation, but rather consist primarily of scattered particles of very small size, on the order of less then 100 nm.
Scanning electron microscopy (SEM) or Auger electron spectroscopy (AES) may be used to detect haze or other sources of transmission changes. However, such methods cannot be used to map a full-sized mask due to their slow speed and small field size. Such methods may only be used to examine defects after they have been detected and their locations determined. In addition, these techniques are destructive, requiring removal of the pellicle, thus obviating the need for the inspection.
One method of local transmission measurement was suggested by U.S. Pat. No. 7,251,033 (Phan et al). This patent describes in situ measurements on a reticle that is located inside a photolithographic exposure tool. However, the method is only able to measure clear areas on the mask surface, and cannot map the full active area of the mask. The method is also limited by its utilization of the light source of the lithographic exposure tool, which is a coherent monochromatic source with a noise level as high as 1%. Therefore, this method cannot detect defects at early stages of their growth, where measurement of transmission with accuracy within a fraction of a percent is required.
Another method, described by U.S. Pat. No. 6,614,520 (Bareket et al), detects defects that result in scattered light. This method involves capturing an image of the light scattered by a known good reticle, and comparing the image pixel by pixel with images captured from the reticle at later times. An algorithm is suggested for determining the thresholds of changes in light intensity that are to be used in identifying defects. This method is sensitive to individual features of the mask, and requires the scanning and detection of individual features on the reticle in order to enable distinguishing between defects and features.
It is an object of the present invention to provide a method and apparatus for metrological transmission mapping of an object that is at least partially transparent to DUV, such as for example a patterned photomask, a blank substrate (e.g. fused silica substrate) and a MoSi coated blank, in order to detect irregularities in the transmittance of DUV through the object or changes in the transmittance, indicative of defects that need fixing.
Another object of the present invention is to provide such method and apparatus in which the actual detailed features (e.g. specific pattern on a photomask) of the object are not specifically imaged, rendering the detection of transmission irregularities or transmission changes and their evaluation simpler than in methods and systems that are concerned with the details.